Cobalt atoms react spontaneously with diazomethane in solid argon at 12 K to yield CoCH2, N2CoCH2, and at least two (N-2)(x)CoCH2 complexes. CH3CoH is formed when dihydrogen is cocondensed as a ternary reagent. Photolysis of CH3CoH (lambda greater than or equal to 400 nm) gives the sigma-complex Co(CH4) as a spectroscopically detectable species. Photolysis of this complex using an ultraviolet source regenerates CH3CoH. The sigma-complex has the triply degenerate v4(f2) absorption of methane at 1305.3 cm(-1) split into two peaks, suggesting C-3v symmetry. Studies using isotopically labeled methane show that these sigma-complexes do not have rigid structures. UV photolysis of a Co/H-2/Ar matrix at 12-15 K gives CoH2. Co(H-2) complexes could not be detected. Photolysis of Co/D-2/Ar and Co/DH/Ar matrices gave CoD2 and CoHD, respectively.